Magnetic sensors are used in many different applications. One important application is for validating banknotes. A banknote typically contains one or more covert security features to enable the authenticity of the banknote to be validated.
One type of covert security feature is magnetic printing on a banknote; another type of covert security feature involves embedding magnetic features into a banknote. For either of these security features, it is important to be able to detect the magnetic properties of that security feature.
In addition to providing basic magnetic printing, some covert security features incorporate a mixture of high coercivity and low coercivity magnets. As is known to those of skill in the art, the coercivity of a magnet represents the intensity of applied magnetic field needed to reduce the magnetization of that magnet from saturation to zero. High coercivity requires a strong magnetic field to be present to reduce the magnetization to zero; whereas, low coercivity only requires a weak magnetic field to be present to reduce the magnetization to zero.
It is known to use magnetoresistive (MR) sensors to detect magnetic security features, but MR sensors only detect a change in magnetic field strength, such as a transition from a magnetic material to a non-magnetic material, and vice versa. They do not detect the strength of a magnetic field, per se.
Furthermore, MR sensors are not very effective at distinguishing between high coercivity magnets and low coercivity magnets, so any security feature comprising a spatial arrangement of high coercivity and low coercivity magnets may not be reliably authenticated using an MR sensor. This lack of reliable authentication reduces the usefulness of MR sensors at detecting counterfeit banknotes.
It would be desirable to provide a magnetic sensor that can distinguish between high coercivity and low coercivity magnets, and that can also sense the field strength of a magnet, not just the transition between a magnetic material and a non-magnetic material.